Originally Posted by
Caffeine
It's dust on the glass of your laser units. Your machine is running dirty. Some of that inside-the-machine airborne dust collects and builds up on the glass windows the laser shines through as it is traveling from the laser unit to to drum. Obviously if enough dust builds up that glass, the laser beam (which is of course light) is partially blocked, and if it is blocked, it can't strike and charge the drum properly.
How to clean it, well, you can do it, but it isn't easy to get to. At all. I can't really walk you through it, because I'm not there, but you'll need to pull your drum drawer out, and also remove the IBT belt unit and set it aside, then you can acrobatically squeeze yourself in there and get it. If you don't have proper cleaning wands, you can use q-tips in a pinch. Be careful. There are some YouTube videos on cleaning the laser glass, too, which should help now that you know what it is you are looking for. (Make sure you watch videos for this same model, or a model with he same design, or the instructions may not help. Any DocuColor 240-260, WorkCentre 76xx, WoreCentre 77xx, Color 550-570, C60/C70, 700/770, C75/J75 instruction will be identical. They have identical laser units. (In terms of their design and the process of cleaning them anyway.)
There is one other possibility, and it would be worse. That is if the dust has gotten INSIDE the laser unit and is dusting up the glass on the inside side of it rather than the outside side of it. Or if it is on the mirrors or lenses inside the laser unit. This was MUCH more common on the DocuColor 240-260 and WorkCentre models than the other newer models where they improved the NOHAD system, but it is possible. Unfortunately, this would be much worse. It is an extreme endeavor (especially for a DIYer) to remove and open up the laser units to clean them. They are very fragile, and even Xerox doesn't tell their tech to open them and clean them. They just tell them to replace the laser units. (But even GETTING to the laser units is a massive chore.)
If you have a leaky drum, aligning with the location of the streak, that is your likely source of dust. You'll see toner dust residue on the drum housing in the same location as well. Similarly, if the mylar "flap" on the front of the top cover of your developer units is getting old and stiff, or even has nicks or damage, it will cause dust to kick up and create this problem.
You've got to keep your machine running CLEAN to avoid the hassle of this type of streak. This also means replacing your suction filter frequently. It is this suction fan (and filter) which draws dusty air AWAY from sensitive parts like the laser. If the filter isn't replaced, it gets filled up like ANY vacuum filter, and therefore less air flows through it which means less air "draft" pulling the dust AWAY from the laser units. This filter is one part of the NOHAD system in the machine. (Noise Ozone Heat Air and Dirt) You can clean things up after they get dirty and start affecting the print quality, but it'll just come right back if you don't clean the filters.
Lastly, if you are using generic toners, which are extremely dusty, you are just BEGGING for this kind of streak to appear right away, even on a pristine machine. Generic toners are devil. (Edit: I just saw that you use genuine supplies, that's good. Changing out the drum chips is no problem. That won't cause you any problems, as long as you are changing the chips to convert a NEW drum from one of the other machines to your 700. (And since you have a 700, it must be one of the styles that has the foam roller on top, too. Not the older style.) If you are using chips to extend the life of your drum, it's not worth it. The seals on old drums start leaking. If they've gone a full cycle, even if they're still printing good, they likely leak at that point.)
Hope that's helpful. It should point you in the right direction, but now it is up to take it from there, if you want to try and DIY it. Or at least you can point a tech in the right direction. You don't need any parts if you can just clean it. But if they don't know this machine specifically, they're likely to do more hard than good just "trying" to get in there and do it. (And if they do know this machine and are trained, they 100% absolutely SHOULD have recognized that particular type of streak at first glance with zero hesitation. "Ah, dirty laser window!" To not recognize it immeidately is very suspicious.)
Best of luck!
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